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Oct. 4, 2011 (Miami Beach, Fla.) -- More than three-fourths of people with a variety of cancers have low levels of vitamin D, and the lowest levels are associated with more advanced cancers, a new study suggests.

High-dose supplements increased vitamin D levels to normal in most patients studied, but it is too soon to know if supplementation improved their outlook, says researcher Thomas Churilla, MS, a third-year medical student at Commonwealth Medical College in Scranton, Penn.

The findings were presented here at the annual meeting of the American Society for Radiation Oncology (ASTRO).

Vitamin D and Cancer Risk

Vitamin D is found in some foods, especially fatty fish, milk, and fortified cereals. Vitamin D is also obtained by exposure to sunlight.

Some studies have suggested a link between low vitamin D levels and cancer risk and progression, but others have not. None has proven cause and effect.

Research in the laboratory suggests that vitamin D has anti-tumor properties, regulating genes involved in the multiplication and spread of cancer cells, Churilla tells WebMD.

For the study, the researchers collected blood samples from 160 men and women with cancer and measured their levels of vitamin D. The five most common diagnoses were breast, prostate, lung, thyroid, and colorectal cancers.

Among people in the study, 42% had vitamin D insufficiency, defined as levels between 20 and 30 nanograms per milliliter (ng/mL) of blood. An additional 32% had vitamin D deficiency, with levels less than 20 ng/mL.

Vitamin D Levels and Stage III Cancer

The average level of vitamin D was about 24 ng/mL. People with levels below 24 ng/mL were nearly three times more likely to have stage III cancer than those with higher vitamin D levels.

Criteria for different stages differ by type of cancer. But in general, stage III indicates more extensive disease than lower stages: larger tumor size and/or spread of the cancer beyond the organ in which it first developed to nearby lymph nodes and/or adjacent organs.

There was no association between low vitamin D levels and even more advanced, stage IV cancers that have spread throughout the body, however. Churilla says that could be because patients with stage IV cancers may have already been seen by a variety of doctors who treated them for vitamin D deficiency.

Vitamin D Supplementation

As a second part of the study, the researchers treated patients with low vitamin D levels with 50,000 international units (IU) of vitamin D a week for eight weeks. The recommended dietary allowance of vitamin D for most people is 600 IU a day, but much higher doses are often needed for the short-term treatment of vitamin D deficiency. This type of treatment should only be done under a doctor's supervision.

The average vitamin D levels of the supplemented patients rose to about 35 ng/mL -- in the normal range.

Now the patients have to be followed for months and years to see if vitamin D supplements appeared to reduce the chance of cancer spread and extend lives, Churilla says.

Phillip Devlin, MD, a radiation oncologist at Harvard Medical School, tells WebMD that such a study does not show cause and effect, only an association between low vitamin D levels and stage III cancer.

Churilla agrees. It could be that people with stage III cancer are more likely to have low vitamin D levels because they are sicker and don't eat as well or get out in the sun as much as people with less advanced cancer, he says.

Devlin says the study generates interesting ideas that need to be tested in larger, longer studies.

"We do not recommend vitamin D supplementation for cancer patients at this point," he says.

These findings were presented at a medical conference. They should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.